6. THE LYMAN ALPHA FOREST AND GALAXIES

Is the Ly forest absorption
caused by galaxies ? To match the large rate of incidence of typical
Ly absorbers, "normal"
(i.e., known types of) galaxies must possess very large absorption
cross-sections
(Burbidge et al 1977).
Alternatively, since the rate of
incidence is constraining only the product of number density and
geometric cross-section of the absorbers, there could be a population
of unknown, more numerous objects, subtending a smaller cross-section.
Two key observations have fuelled the interest in the nature of the
absorber-galaxy connection. One was the detection of apparently
ordinary galaxies at the same (intermediate) redshifts as high column
density, MgII metal absorbers
(Bergeron 1986).
Subsequent work
(Bergeron & Boissé
1991;
Steidel 1995)
established an
incontrovertible connection between gaseous galactic halos and high
column density, metal absorption systems (which we will not discuss here,
as it is most relevant for optically thick Lyman limit systems). The
other was the widely unanticipated detection of a remnant population of
Ly absorbers in the
local universe with HST
(Morris 1991;
Bahcall et al 1991,
1993),
where the
properties of an galaxy can be studied in much greater detail than at
high redshift.

Soon after the discovery of the low z absorbers, galaxy surveys in the
fields of the QSOs observed by HST, especially near 3C273, were
undertaken to investigate a possible link between absorbers and galaxies.
Salzer (1992),
Morris et al (1993) and
Salpeter & Hoffman
(1995)
found redshift
coincidences with galaxies for some of the lines, hundreds of kpc up
to Mpcs away from the absorbers, but there was no unambiguous
correspondence between absorbers and individual galaxies. Searches for
any low surface brightness objects closer to the LOS to 3C273 which
could have escaped detection, have been performed in HI radio emission
(van Gorkom et al 1993),
H emission
(Morris et al 1993,
Vogel et al 1995),
and in deep broad band optical images
(Rauch et al (1996),
but were all equally unsuccessful.
Morris et al (1993)
concluded that there is some correlation between absorbers and
galaxies, but the galaxy-absorber cross correlation function is weaker
than the galaxy-galaxy correlation. By trying to model the correlation
results as a mixture of randomly distributed objects and galactic halos,
Mo & Morris (1994)
deduced that galaxy halos constitute only about
25% of the local absorber population. At first glance this is at
variance with the result of
Lanzetta et al (1995),
who, based on their large galaxy
redshift survey, concluded that most absorption systems are associated
with galactic envelopes of typical radius 160 h-1 kpc
(see also
Chen et al 1998).
Moreover, a significant anticorrelation between the
equivalent width and the impact parameter of the LOS from the center of
the galaxies was measured.
Le Brun et al (1996)'s
survey showed a weaker anticorrelation, and
Bowen et al (1996)
saw none at all, although they both confirm Lanzetta et al's results in
that there is a region with a high covering factor for absorption around
each galaxy, within impact parameters
< 200 - 300h-1 kpc, dropping
rapidly at larger separations. It appears that the conclusions of
Morris et al (1991) and
Lanzetta et al (1995)
can be reconciled if the
Lanzetta et al's main result, i.e., the existing of well defined galactic
envelopes, is valid only for the relatively strong absorption lines
used in their comparison. The weaker lines, from which
Morris et al (1991)
derived a weak galaxy-absorber correlation, could still be
caused by a truly intergalactic medium. Absorption systems do exist in
voids known from galaxy surveys
(Stocke et al 1995;
Shull et al 1996),
but there appears to be a general trend for the absorbers to trace the same
large scale structure as galaxies
(Stocke et al. 1995;
Hoffman et al 1995).
In any case, the very large structures causing the
stronger common absorption systems in QSO pairs
(Bechtold 1994;
Dinshaw et al 1994,
1995)
cannot be explained by single objects like giant disks or
halos: in those cases where several galaxies have been found at the same
redshift as the absorber the typical transverse separation between the
galaxies is smaller than the transverse absorber size
(Rauch et al 1996).
This is reminiscent of
Oort's (1981)
"uncondensed gas in superclusters". There is amazing redshift agreement
(velocity differences
0 < v < 20
kms-1) between the HI velocity centroids of galaxies and
absorption systems with impact parameters as large as 300
h-1 kpc
(van Gorkom et al 1996).

MODELLING LOW REDSHIFT LYMAN ALPHA ABSORBERS
Attempts at modelling have mainly been concerned with the large
cross-section required if the low redshift
Ly clouds are parts
of the known galaxy population.
Maloney (1992)
has suggested that the
absorption arises in the distant, ionized outer parts of the known
population of disk and irregular galaxies. Clouds that are pressure
confined by a hotter gas in a extended galactic halo were discussed by
Mo (1994).
Salpeter (1993)
has invoked a new type of extended low redshift disk
galaxies which, in a short burst of star formation blow out their
denser centers and disappear from view, while their gas cross-section
remains (the so-called "Cheshire Cat model"). The large sizes and
low column densities postulated require that these objects must have
formed late
(Salpeter & Hoffman
1995).
Again, observationally the
large coherence lengths of the absorbers on the sky seem to correspond
to groups of galaxies. To get the large covering factor right these
objects must be so close together as to run into each other like
circular saws at only slightly higher redshift, which puts us basically
back at a continuous intergalactic medium. A number of other
sources of low z Ly
absorption has been considered, among them tidal tails
(Morris & Van den Berg
1994),
and galactic winds
(Wang 1995).
Galaxy clusters have been seen to cause at least some absorbers
(Lanzetta et al 1996),
and the strong clustering found among low z absorption lines
(Boksenberg 1995;
Bahcall et al 1996;
Ulmer 1996)
hints at the increasing importance of such structures with decreasing
redshift.

Clearly, one has to be open-minded regarding all these
possibilities, many of which plausibly may contribute a significant
fraction to low z Ly
absorption. How much exactly may be hard to quantify
(Sarajedini et al 1996).
Again, believers in hierarchical structure formation can expect some
consolation from simulations.
Petitjean et al (1995)
predicted a bi-modal distribution of absorbers, with large galaxy halos
with typical radii on the order of 0.5 h-1 Mpc on one
hand, and more frequent,
lower column density intergalactic absorption occuring in filaments up to
several Mpc away from the nearest galaxy, on the other. Miraculously,
this is consistent with all the observational evidence we have.